This invention pertains to the art of ceramic powder precursors, and more particularly to a method for producing coprecipitated multicomponent oxide powder precursors.
The invention is particularly applicable to a method for coprecipitating metal oxalates as precursors for multicomponent oxide powders used in producing ceramics for a variety of applications, and will be described with particular reference thereto. It will be appreciated, however, that the invention may be advantageously employed in other environments and applications.
The coprecipitation of mixed salts from liquid solutions is a well-established method of ceramic powder precursor synthesis. Coprecipitation refers to the simultaneous precipitation of more than one metal from the same solution.
A multicomponent liquid solution of soluble inorganic salts (e.g., metal nitrates, halides, sulfates) is typically combined with a liquid solution of a precipitating agent compound. The precipitating agent is chosen such that, when dissolved and combined with the metals solution, one of its radicals combines with the metal ions to form insoluble salts which thermally decompose to form oxides. The insoluble salts will precipitate in a very finely divided and intimately mixed state. Heating the precipitate decomposes these salts, resulting in a chemically homogeneous, fine oxide powder with high surface area. This powder may then be fabricated into a number of ceramic products using various ceramic fabrication techniques. Examples of such ceramic products include, but are not limited to, electrical or electronic ceramics (integrated circuit substrates, capacitors, piezoelectric transducers, ferroelectric devices, or optical or optoelectronic devices, solid electrolytes, electronically conductive ceramic electrodes, and ceramic superconductors); magnetic ceramics (magnetic storage media, video or audio tape heads, transformer cores, memory devices or arrays); ceramics used primarily for their strength, hardness and/or chemical stability (refractories; heat exchangers; abrasives; fibers for reinforcement; bulk materials and coatings for protection from heat, oxidation, corrosion, wear, stress, or other physical or chemical changes; catalyst substrates); pigments; and catalysts.
The type of precipitate formed depends on the precipitating agent used. The precipitating agent can be selected from among a variety of compounds including, as examples, hydroxides, carbonates, and oxalates. Although there are advantages and disadvantages to using each of the various types of precipitating agents, precipitated carbonates and hydroxides in many cases tend to be gelatinous thereby difficult to rinse, separate, and filter.
As a class, oxalates are generally highly insoluble, and they form particles that are readily filtered from the liquid and easy to handle. For example, oxalates of the following compounds exhibit low water solubility: Al, Ba, Bi, Cd, Ca, Ce(III), Cr(II), Co, Cu, the rare earths, Ga, Fe(II), Pb, Mg, Mn, Hg, Ni, Ag, Sr, Tl(I), Th, U, Y, and Zn.
Coprecipitated oxide powder precursors offer increased homogeneity as well as increased reactivity over those precursors which are not coprecipitated. While coprecipitation is not the only way to achieve these advantages, the increase in homogeneity and reactivity is especially advantageous in multicomponent oxide systems where the attainment of solid-state equilibrium is often slow, or when reactions must be carried out below a melting temperature. These conditions prevail in the cuprate superconductors, for example, and several coprecipitation routes for their synthesis have been reported. Coprecipitation has also been used to produce magnetic oxide materials.
In current coprecipitation procedures, many of the precipitated salts exhibit slight solubilities in the supernatant liquid. This leads to incomplete precipitations.
To insure a complete precipitation and the precise cation stoichiometry desired, the pH of the mixture is controlled. In light of this, there are several disadvantages associated with the prior art methods of coprecipitation.
First, if alkali metal hydroxides (e.g., NaOH, KOH etc.) are used to adjust the pH of the precipitating solution, extensive washings of the precipitates are necessary to remove alkali metal residues which remain as contaminants in the final mixed oxide product. Similar contamination of the powder by alkali metals can result from using alkali metal oxalates or carbonates (e.g., Na.sub.2 C.sub.2 O.sub.4 or Na.sub.2 CO.sub.3) as precipitating agents.
Second, if aqueous ammonium hydroxide is used to neutralize the pH, or if ammonium oxalate is used as a precipitating agent, water-soluble ammonia complexes of certain ions (e.g., copper, nickel or silver) can form.
Third, if weak organic bases are used, it is difficult to achieve a high enough pH required to quantitatively precipitate many oxalates unless large quantities of the weak base are used. In some cases, because of their relatively high molecular weights, the large amount of weak base needed becomes impractical. As is the case with ammonia, many weak organic bases form soluble amine complexes with some cations such as Cu.sup.++ and Ni.sup.++. This tends to prevent quantitative precipitation of these cations.
Fourth, of the moderately strong organic bases, the tertiary amines, only trimethylamine, the first in the series, is practical in terms of a convenient equivalent weight. The equivalent weights of the tertiary amines increase rapidly upon going up in the series to triethylamine and tripropylamine. Trimethylamine, however, offers a few disadvantages in that it is expensive to obtain and, in addition, has three carbon atoms per molecule which may leave a carbon residue, especially on firing in a nonoxidizing atmosphere.
Of the very strong bases, the quaternary ammonium hydroxides, about equal in strength to sodium hydroxide, only the first in the series, tetramethylammonium hydroxide, is practical. Tetramethylammonium hydroxide, nonetheless, has drawbacks similar to those found in using trimethylamine.
Fifth, a pH adjustment, subsequent to the initial precipitation, leads to a second precipitation. The second precipitate would probably have a different composition than the first. This could lead to some degree of segregation of the metals within the precipitate, as well as a loss of compositional homogeneity. For homogeneity of a coprecipitate, a single one-time precipitation is advised.
Sixth, the solutions can be chilled or mixed with alcohol to decrease the solubility of the precipitates. The alcohol can also act as an antifreeze enabling precipitations below 0.degree. C. However, several precipitating agents have lower solubility in cold water or are insoluble in alcohol. Examples of these precipitating agents include the oxalates of sodium, potassium, and ammonium.
It would be desirable to develop a method for quantitatively coprecipitating multicomponent, chemically homogeneous oxide powder precursors.
It would be further desirable to develop a method for coprecipitating multicomponent oxide powder precursors such that upon decomposition, intimately mixed, finely divided oxides with high surface area and chemical homogeneity would be produced.
The present invention demonstrates a new and improved method which addresses the above-referenced problems and others, and provides a method for coprecipitating multicomponent ceramic powder precursors that is simple, quantitative, and economical.